Hydraulic or oil pressure control systems, which are used in oil circuits for driving actuators for stationary or mobile machines, may sometimes be subjected to sudden changes in pressure. For instance, when activating or starting up a hydraulic system from an inactive state an abrupt increase in pressure may cause a pressure pulse, sometimes referred to as a hydraulic ram. Although such pressure pulses are generally not a problem for hydraulic devices or valves in the system, but may cause undesirable noise and/or vibrations that are noticeable to an operator.
An example of a hydraulic system that may give rise to such problems is shown in FIG. 1. The system comprises a source of high pressure in the form of an accumulator A connected to a non-specific consumer C via a flow control valve 1. The consumer may be a hydraulic cylinder, a hydraulic pump/motor or any such device interacting with hydraulic pressure. Actuation of the flow control valve 1 is controlled by a solenoid valve 2 in the form of a standard two-position solenoid operated valve. The flow control valve 1 comprises a poppet 3 that is spring loaded on by a spring 4 in the direction of a closed position of the flow control valve 1. As shown in the figure, poppet 3 prevents flow between an input/output port 5, connected to the accumulator A, and an output/input port 6, connected to the consumer C. In this context, the term “input/output port” is used for ports where the main direction of flow is from a source of pressure to a load, but where the direction is reversed under certain conditions. Similarly, the term “output/input port” is used for ports where the main direction of flow is from a load to a source of pressure. FIG. 1 shows the system with the solenoid valve 2 held in its non-actuated position by a spring load, wherein the accumulator A is connected to and pressurizes the side of the poppet 3 acted on by the spring 4. This side is referred to as the spring side 8. When the solenoid valve 2 is held in its actuated position, the spring side 8 is instead connected to the tank T.
In operation with the flow control valve 1 in its inactive state, the flow control valve 1 is maintained in its closed position by high pressure from the accumulator A and the spring 4 at the spring side of the poppet 3 in the flow control valve 1. Under transition from active to inactive state of the flow control valve 1, the sum of forces created by the pressure from the accumulator A acting on the input/output port 5 and any pressure from the consumer C acting on the output/input port 6 will be less than the force created by the pressure from the accumulator A acting on the spring side 8 of the poppet 3. Over time, internal leakage through the consumer C, indicated as a throttle 7 between the consumer and the tank T, will cause the pressure at the consumer C to drop to tank, or reservoir, pressure.
In order to operate the consumer C with pressurized hydraulic fluid from the accumulator A, the solenoid valve 2 is actuated in order to pressurize the said consumer C. When the solenoid valve 2 is displaced to its actuated position, hydraulic fluid acting on the spring side 8 of the poppet 3 in the flow control valve 1 is drained to the tank T through a damping throttle 9. High pressure from the accumulator A at the input/output port 5 acting on a poppet ring area of the poppet 3 opens the flow control valve 1. The relatively high pressure difference across the flow control valve 1 causes a relatively abrupt rise in pressure in the consumer C.
An inherent feature of a flow control valve of this type is that a relatively small displacement of the poppet to open the valve will open up a relatively large flow area. The abrupt pressure rise in the flow control valve 1 creates an uncontrolled pressure transient in the consumer, causing a distinct noise similar to a fluid hammer. Immediately after opening, a pressure pulse caused by the pressure transient may cause the pressure in the consumer C to be higher than the pressure in the accumulator A. The damping throttle 9 will only have a limited effect on the rate at which the hydraulic fluid is drained from the spring side 8 and can not eliminate this noise.
A further problem that may occur in hydraulic or oil pressure control systems is a sudden loss of pressure in a consumer or actuator. In the example shown in FIG. 1 the consumer may be, for instance, a hydraulic device that is connected to a supply of hydraulic pressure in the form of an accumulator, as shown in FIG. 1. A sudden loss of pressure in the consumer with a subsequent uncontrolled flow of hydraulic fluid from the supply of hydraulic pressure through the flow control valve may, if not checked, cause damage to the accumulator.
Alternatively, the consumer C may be a hydraulic pump/motor. Under certain conditions, such as a sudden overload of the pump/motor, hydraulic fluid may leak from the cylinders of the pump/motor into the housing surrounding the pump/motor. If the flow of hydraulic fluid is interrupted, the hydraulic pump/motor may resume operation after the excess fluid has been drained out of the said housing. Should the flow of fluid continue, then the pressurized fluid may cause the housing to burst, requiring substantial repairs to the hydraulic pump/motor. The prior art arrangement as shown in FIG. 1 has no means for detecting excessive flow or for interrupting such a flow of hydraulic fluid.
A common way of solving this problem is to provide the system with a hose burst valve. However, this solution requires the mounting of an additional valve in the system and increases the complexity, weight and cost of the system.
One object of the invention is to overcome the above problems by providing an improved hydraulic system that will minimize generation of undesirable noise and/or vibrations caused by pressure pulses. A further object of the invention is to provide an improved hydraulic system that will prevent an uncontrolled flow of hydraulic fluid from the supply of hydraulic pressure caused by a sudden loss of pressure in the consumer.